How to "letterbox" a video to a wider aspect ratio?
Set the render dimensions to what you want the output video to be, for example 1920x1080.

In the VSE, add an Effect Strip of type Color as channel 1, and adjust its length to match the video (you can go back and adjust this after adding the video, in the next step)



Add the video to channel 2, and set Blend to Alpha Over. Enable Image Offset and adjust the offset until the video is properly placed. The cropping that you're also doing, works well together with the offsetting, just remember to use the cropped size, not the original size, when you work out the offset.